Conventionally, as shown in the patent document 1 or the patent document 2, a dilution air refinery used for an analysis system of a measurement object gas such as an engine exhaust gas oxidizes CO, HC, NOx or the like by the use of an oxidation catalyst (Pd system, Pt system or the like) and adsorbs and removes NOx by an NOx remover.
Meanwhile, in order to remove HC, CO, NOx or the like in the exhaust gas, a Pt catalyst or a Pd catalyst (a three-way catalyst) is used, and purification of especially NO and CO is explained by a reaction of 2CO+2NO→2CO2+N2 (reaction 1). However, depending on each catalytic condition such as concentration balance, it is known that a side reaction of CO+2NO→CO2+N2O (reaction 2) often happens and N2O is produced. After the reaction 2, when CO+N2O→CO2+N2 (reaction 3) is produced continuously, the reaction becomes equivalent to the above-mentioned reaction 1.
Recently, dinitrogen monoxide (N2O) has drawn attention as a greenhouse gas. N2O contained in the exhaust gas is also no exception so that a need for analyzing a concentration of N2O contained in the exhaust gas arises.
In case that the exhaust gas sampling analysis system analyzes the exhaust gas by the use of a dilution sampling method, a concentration of a component in the dilution air is measured and a concentration of a component in the diluted exhaust gas is also measured, and a correction of a background is conducted by evaluating the difference between the measurement results, and finally a concentration of a component in the exhaust gas is calculated. The same method is used also in case of measuring a concentration of N2O in the exhaust gas.
However, N2O of about 300 [ppb] is already contained in the dilution air collected from the atmosphere. Then especially in case that the concentration of N2O contained in the exhaust gas is approximately equal to or less than 300 [ppb], there is a problem that the measurement accuracy of the concentration of N2O in the exhaust gas is aggravated because the measurement error of the concentration of N2O of the dilution air significantly affects a concentration of N2O in the exhaust gas after the correction of the background. In other words, there is a problem that a ratio of the concentration of N2O in the dilution air to the concentration of N2O in the diluted exhaust gas is big.